1. Field of the Invention
The present invention relates, in general, to electromechanical servo and other positioning systems and methods for use with magnetic hard disk drives (HDDs), and more particularly, to a position control system and method for disk drive servo positioning in an HDD system that utilizes two mechanically connected but separately positionable actuator assemblies in a dual stage configuration to position a read-write head relative to a magnetic disk.
2. Relevant Background
Millions of HDD systems are produced every year and competition for the revenue from the sale of these HDD systems has resulted in a demand for cost effective and highly accurate read-write operations. To read or write data on a spinning magnetic disk requires accurate dynamic, ongoing positioning of a read-write head in a HDD system relative to a desired track on the magnetic disk. Because measuring the actual position of the head is difficult and expensive to achieve, HDD system manufacturers have dispensed with position sensing. Instead, HDD systems presently use position control systems that position the head using relative track position information obtained or read directly from data contained on the disk itself.
For many conventional HDD systems (such as the HDD system 10 shown in FIG. 1), the absence of this measurement is of minor importance because the actual track position varies in space due to runout. As shown, a conventional HDD system 10 may include an actuator assembly 12 with a pivoting actuator 14 that positions a read/write head 16 by positioning or moving an actuator arm 18 relative to tracks on the magnetic disk 20. Often, the actuator 14 is a current driven voice coil motor (VCM). Runout is the deviation from perfect concentric circles of the positions of the tracks on the magnetic disk 20 and includes deterministic or repeatable runout and random or nonrepeatable runout. Due to the existence of runout, the relative position of the head 16 with respect to a track on the magnetic disk 20 is more important and useful for controlling the position of the head 16 by operating the VCM actuator 14 for reading and writing of data.
The track position data read or obtained via the head 16 is contained in specific areas of the magnetic disk 20 surface called servo wedges 22. There are typically fifty to one hundred equally spaced servo wedges 22 on the surface of each magnetic disk 20. The position data includes track number (and/or cylinder number for multiple disk systems 10) and burst information that quantifies the position offset of the head 16 from a center line of the track currently being read by the head 16.
A conventional position control system 30 is shown in the block diagram of FIG. 2. The position control system 30 utilizes a feedback position control approach to position the head 16 relative to a particular track on the magnetic disk 20. As shown, an input command signal or input variable 32 is provided to a VCM controller 34 which processes the signal 32 and passes a useful control signal 36 to the mechanical system 38 (such as actuator assembly 12). The mechanical system 38 further processes the control signal 36 to address such variables as nonlinear friction 40, torque bias and viscous friction 42 and then acts to position the actuator arm 18 and read/write head 16 along the track center line as commanded by the input set point command 32. The position control system 30 (e.g., the HDD electronic system) processes the track number and center line offset in signal 44 read by the head 16. The effect of runout is modeled by adding a disturbance in the form of a signal 46 to the (unmeasured) absolute position in the feedback section to form a composite position error signal (PES) 48. The PES 48 is fed to the VCM controller 34 as feedback that is used to update and/or correct the position of the read/write head 16. The actual angular position, ypa 50, is not measured by the control system 30.
To provide a competitive edge, HDD systems are being designed and configured with much higher track density to allow storage of more data on similarly-sized magnetic disks. While providing more data storage, these higher track densities have created problems in controlling the position of the read/write heads rapidly with acceptable accuracy. High track density HDD systems place high demands on the control system for resolution and the control system needs to have sufficient bandwidth or capacity to provide useful rejection of disturbances, such as mechanical vibration.
In HDD systems, such as the system 10 shown in FIG. 1, the resonant structural modes of the actuator arm 18 are excited by rapid acceleration during slewing (i.e., a rapid change of position when the system 10 moves at full speed from one track position to another) and rapid decelerations while braking to a stop. Excitation of resonant structural modes causes mechanical vibration resulting in unwanted movement of the head 16 that makes accurate tracking of the position of the head 16 very difficult, especially in the presence of runout. Due to the stiffness and length of the actuator arm 18, conventional VCM-based actuator assemblies 12 such as that shown in FIG. 1 can have very high resonant frequencies, e.g., in the kilohertz range, that require bandwidths in control systems to be large, such as 500 to 800 Hertz. Even with these larger bandwidths, existing control systems are often incapable of quickly and accurately controlling head position. Hence, HDD system designers and manufacturers continue to demand new actuator assemblies to overcome vibration problems. However, changes to the actuator assemblies create new and often unexpected challenges in providing a system and method for controlling the position of the head relative to the magnetic disk.
Hence, there remains a need for an improved method and system for identifying and controlling the positioning of a read/write head relative to tracks on a magnetic disk. Preferably, such a system would enable high performance track following of magnetic track position in the presence of runout and at least in some embodiments, provide high accuracy of positioning of a head without the use of external motion and/or position sensing devices.
The present invention addresses the above discussed and additional problems by providing a position control system and associated method for providing accurate and efficient control over an actuator assembly in a disk drive system. The actuator assembly includes a primary actuator, such as a VCM actuator, with a large stroke for positioning or pivoting a primary actuator arm to obtain rapid and larger movements to rapidly position a head near a desired track on a disk. The actuator assembly also includes a secondary actuator with a small stroke mounted pivotally on the primary actuator arm for positioning a secondary actuator arm that supports the read/write head over the centerline of a desired track. The position control system includes a primary actuator controller, a secondary controller and a feedback system that in combination are uniquely adapted for independently controlling each of the actuators to overcome prior difficulties with mechanical vibration, saturation and locking of the secondary actuator.
More particularly, a position control system is provided for use in a disk drive system for positioning a head relative to a storage disk in response to an input command signal. The disk drive system includes a primary actuator for positioning a primary actuator arm and a secondary actuator linked to the primary actuator arm for positioning a secondary actuator arm on which the head is mounted. The position control system includes a feedback system or circuit for receiving the input command signal, for splitting the input command signal into a primary and a secondary input command signal and for modifying the primary and secondary input command signals to produce primary and secondary error signals.
A primary actuator controller is linked to the feedback system and the primary actuator. This controller receives the primary error signal and in response, transmits a primary control signal including primary actuator arm positioning information to the primary actuator. Additionally, a secondary actuator controller is linked to the feedback system and the secondary actuator configured to receive the secondary error signal and in response to transmit a secondary control signal including secondary actuator arm positioning information to the secondary actuator. In one embodiment, the feedback system is further adapted for creating a position error signal (PES) using position information obtained from servo wedges in the storage disk modified by sampled runout information. The PES is then used by the feedback system to produce the secondary error signal.
According to an important aspect of the invention, the feedback system includes a spatial position reconstruction mechanism for producing a reconstructed error signal comprising angular position information for the primary actuator arm. The reconstruction mechanism synthesizes the reconstructed error signal by processing an operating parameter, e.g., a back electromotive force signal, of the primary actuator. The primary error signal is then produced by modifying the primary input signal with the reconstructed error signal. In one embodiment, the feedback system further includes a settle detection element for determining when the primary error signal has a magnitude less than a threshold value and when xe2x80x9csettlingxe2x80x9d has occurred, an output signal is transmitted to a switch in the secondary actuator controller to begin positioning of the secondary actuator arm.